The invention concerns the field of the transport of cold products in ducting, in particular in the field of foodstuffs.
In various industries, in particular the food industry, which will be taken as a preferred but not exclusive example in the rest of the description, there is a need to transport frozen products through ducting so as to deliver them to a treatment or storage site (including sites where wrapping or packaging operations are carried out). These frozen products can be in viscous form or in the form of a divided solid. As a product in a viscous form, mention may be made of soups. As products in a divided solid form, mention may be made of peas and other vegetables.
These transport installations generally operate in a manner that is as continuous as possible. Nevertheless, it is inevitable that their operation experiences transient periods: stopping and restarting of production, or a product change when the same transport installation is connected to several installations for the production or storage of different products. During these transient periods, the ducting becomes heated and with it the product that may be located in it. This heating of the product can thaw it and therefore render it unusable. When production starts again this product must then be rejected, with all the economical disadvantages that this entails.
In the case where no product is located in the ducting, in particular at the moment when the installation starts, the first quantities of product passing through the ducting are heated by contact with it and, in this way, can become unusable. They will have served only to bring the ducting to its normal working temperature. Moreover, since loss of the product has served to cool the ducting, the disadvantages of this procedure are an increase in rejects from the installation that have to be retreated, and the risk of fouling and contamination of the ducting by fractions of thawed product that would remain therein, with all the problems of hygiene that this can entail. It will thus be seen that this simple solution to be implemented to bring the ducting to its working temperature is however unsatisfactory from an economical point of view and from a technical point of view.
Another possible solution is to provide so-called “jacket” ducting and to circulate a cooling liquid such as ammonia at about −35° C. around the ducting for transporting the product, at least during periods when production is interrupted. This solution nevertheless presents technical problems. It makes the ducting more complex, which complicates disconnection and reconnection operations during cleaning and replacements that have to take place regularly, in particular when ducting with different diameters has to be used according to the nature of the transported products. In addition, it is not possible to cool the mechanical elements of the installation in this way such as pumps or other devices ensuring progression of the product and to which the different sections of the ducting are connected. Now, passage through these mechanical elements that are not yet cold is often responsible for the major part of product heating. The fact should also be pointed out that the product itself will in this way bring about soiling of these mechanical elements.
The object of the invention is to provide a satisfactory solution, both from the technical point of view as well as from the economical point of view, to the problem of the reheating of frozen products during transient periods, as has just been described.